Noise-shaping, over-sampled data converters realize processing gains in effective dynamic range by accepting increased conversion time in order to realize higher conversion accuracy. Accuracy is achieved by employing an over-sample rate (OSR) that is greater than the Nyquist sampling rate (twice the signal bandwidth). Furthermore, multilevel feedback paths may be incorporated to further extend dynamic range. Specifically in a sigma-delta analog-to-digital converter (ADC), multilevel feedback may be implemented in the form of a multibit digital-to-analog converter (DAC). When multibit feedback is applied to a sigma-delta data conversion system, the OSR can be reduced, thereby relaxing the burden on the system with respect to analog element bandwidth, settling requirements, and power consumption. Consequently, use of multilevel feedback allows higher signal bandwidths to be processed at an effectively lower conversion rate, while producing a similar dynamic range.
However, multilevel feedback in a sigma-delta data conversion system has heretofore proven somewhat intractable in its implementation. Specifically, the linearity requirement imposed on the multilevel (i.e., multibit) feedback DAC is generally required to be of the same order as the dynamic range realized by that ADC system. For example, a third-order sigma-delta ADC realizing an 86 dB dynamic range (corresponding to effective 14-bit quantization) requires that the multibit feedback DAC (perhaps 3 to 4 bits of quantization) similarly satisfies a linearity requirement of 14 bits. A feedback DAC that consistently demonstrates such a high degree of linearity has proven daunting in implementation. A number of approaches have been attempted, with only qualified success. For example, in one approach linearity in the feedback DAC has been sought through the imposition of continual calibration. This approach to calibration is typically predicated on dynamic element matching using precision passive components, such as well matched, highly linear, resistors or capacitors. However, the requisite level of passive matching may be had only through devices that occupy significant amounts of semiconductor real estate and that nonetheless tend to be sensitive to process variations.
Accordingly, what remains to be provided is a highly linear feedback stage for a sigma-delta data converter. The feedback stage should preferably obviate the need for time-consuming, repetitive calibration and voluminous precision components. In addition, simplicity of implementation and minimization of power consumption constitute salutary characteristics of the feedback stage.
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